The invention relates generally to electromagnetic flowmeters, and more particularly to an improved insulating liner for the flow tube of such meters.
An electromagnetic flowmeter is adapted to measure the flow rates of those fluids which present difficult handling problems, such as corrosive acids, sewage slurries, detergents and the like. In a flowmeter of this type, the liquid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by a pair of oppositely-disposed electromagnets. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator, or to supply an input variable to a process control system.
It is common practice in flowmeters of this type to install an electrically-insulating protective liner on the inside surface of the metal flow tube. This liner may be cemented in place or the entire internal layer may be vulcanized in place, or the liner may consist of a continuous vitreous enameled coating. Often used as insulating liner material is a fluorocarbon material such as PTFE, PFA, FEP or ECTFE. These fluorocarbons, while they have desirable liner properties, are incapable of adhering to the inner surface of the flow tube for prolonged periods.
It is also known to apply an insulating layer to the inside surface of the flowmeter tube by spraying or thermally melting the material thereon. Liners produced by these methods are non-crystalline in structure and have poorly defined electrical insulating properties. Moreover, such liners are readily rendered porous, so that in the course of time the metallic meter tube is caused to corrode.
Another known approach is to install an insulating liner within the flow tube by employing tube-like materials or by pressing in tube-like materials isostatically. In this case, there is no bonding of the tube-like material to the inside surface of the flowmeter tube. If fluorocarbons are used for this purpose, then damage to the liner material may occur as a result of cold flow of these materials when under load by reason of their large thermal expansion coefficients. In particular, due to stresses induced by pressure or vacuum, folds are developed in the liner at points therein where changes in section occur when the temperature is raised. And when operating under vacuum the tubing may possibly collapse.